This application claims priority benefits of Norwegian Patent Application number NO 2001 3264 filed Jun. 29, 2001.
The present invention relates to a method and a system for retrieving and leading of an underwater buoy into a well at the bottom of a dynamically positioned vessel, wherein the buoy by the use of winching device on the vessel is pulled into the well via pulling line connected to the buoy.
Previously known buoy loading system which comprises the underwater buoy which is pulled in and locked in a well in the bottom of a vessel, is the so-called STL-system (STL=Submerged Turret Loading). This system is based on a submerged buoy which comprises of a central unit anchored to the seabed and the other buoyancy part which has a turnable bearing, which is adjusted in shape to fit the shape of the well and to be loosely attached to this.
STL-system is today using a floating hawser system consisting of a frontline and a messenger line for connection of the buoy and the current tanker. This known arrangement is shown in FIG. 1 where it is shown a STL-buoy 1 with a riser 2 and with anchor lines for anchoring to the seabed. The buoy is via a crowfoot 4 connected with a hawser/messenger line 5 and a frontline 6 where the free end of the frontline is connected to a marking buoy 7. The floating hawser system 5, 6, which has a horizontal extension of the surface of approximately 200 meters, is normally retrieved by tender vessel. The tender vessel subsequently transfers the frontline to the tanker which pulls in the STL-buoy into the vessel""s well by use of a dedicated winching device.
To ensure that a hawser system always shall have a positive buoyancy, it is necessary to use a hawser/messenger line with a special material quality and integration of buoyancy elements.
The known system have the following disadvantages:
A tender vessel will normally be necessary when the tanker shall connect to the STL-system
Often it will the tender vessel which due to safety reasons (people who work on the deck) is limiting a connection operation due the weather condition
The hawser system which is in use today, is costly and require relatively frequent replacements per STL-system per year with normal use)
It is experienced several times that ships set the floating hawser in the propeller with consequences for both the ship and the field operator
The object for the present invention is to provide a method and a system to eliminate the above mentioned disadvantages, and which makes the procedure of connecting a buoy to the current vessel more efficient.
To achieve the above mentioned objectives it is provided a method of the initially mentioned type which according to the invention is characterized by the features which is given in the characterizing part of claim 1.
According to the invention it is also provided a system of the given type which is characterized by the features which is given in the characterizing part of claim 2.
By using the method and the system according to the invention a series of advantages are achieved which can be summarized as follows:
Tender vessel is not necessary
Extension of operational limits can be achieved (since the tender vessel today to represents the limitation
Potential for more efficient operation, i.e. saved tanker time etc.
Reduced maintenance cost (replacement of worn out messenger lines represents today a considerable operational cost)
No danger for a ship to get the messenger line in the propeller